Fuel cells or stacks of fuel cells are known. The individual fuel cell itself has a stacked structure comprising an electrolyte arranged between its outer plates. There is an anode between the electrolyte and one outer plate and a cathode between the electrolyte and the other outer plate. Solid and liquid electrolytes are known; the electrolyte can be held by a carrier structure or may itself have the required strength to allow it to be installed in the fuel cell, depending on whether a solid or liquid electrolyte is used. The operating temperatures also differ considerably, ranging from ambient temperature to several hundred degrees C. and above.
In one of the many known designs to which the invention can be applied, a polymer electrolyte membrane (PEM) is used in the fuel cell. In operation, then, by way of example hydrogen in gas form is fed to the anode side of the fuel cell and, for example, oxygen-containing ambient air is fed to the cathode side of the fuel cell.
In the context of the present description, for the sake of simplicity all the reaction partners which participate in the chemical reaction in a fuel cell are referred to as fuels, as are their carrier fluids (e.g. ambient air as a carrier fluid for the reaction partner O2). The fuels are in fluid form. As has already been mentioned above, there are numerous embodiments of fuel cells with a very wide range of fuels, and the PEM fuel cell which is explained in more detail in the present context serves merely as an example which helps to explain the conditions.
In general, structures which serve as passages for the fuels to pass through are provided in the outer plates of the fuel cell, in such a manner that the electrolyte or membrane is covered with the fuels as uniformly as possible over the largest possible part of its area, which leads to the desired electrolytic reaction and therefore to the generation of current.
The sealing concept of the fuel cell is crucial, since if the sealing is inadequate, the reaction of the fuels as they mix with one another will be uncontrolled. In the case of the PEM fuel cell using hydrogen and oxygen as reaction partners, a leak leads to the detonating gas reaction. In general, special cord seals are used to reliably keep the fuels within their active regions. Of course, other sealing concepts are possible.
The individual cell generates only a relatively low voltage; by connecting the cells in series in a stack, it is possible to achieve a voltage which is adequate for the intended purpose and therefore sufficient power from the stack of fuel cells. Stacks of one hundred or more fuel cells are usual. Nowadays, stacks of PEM fuel cells with around a hundred cells, a power of 7 kw and a weight of approx. 20 kg are known.
In the stack, passages which run along the stack are used to supply the individual fuel cells with the fuels. Special cooling is often also provided, which likewise leads to passages for the coolant to flow to the individual fuel cells. Moreover, cooling passages are then to be provided in the individual fuel cell, and these in turn have to be kept sealed.
The result is that the routing of the media (fuels, coolants, etc.) in the stack requires special design precautions as well as the sealing concept. The passages for supplying the media are often integrated in the outer plates of the individual fuel cell.
Finally, it should be taken into account that the series connection of the fuel cells in the stack leads to the flow of current which is generated by the stack flowing through the stack itself, i.e. from fuel cell to fuel cell, in each case through the outer plates thereof, which are in contact directly or via intermediate layers.
Therefore, the contact resistance between the elements which are in contact with one another becomes critical for the power of the stack of fuel cells: in the abovementioned, standard stack, there are in each case a hundred electrodes and outer plates, resulting in several hundred contact surfaces with a corresponding contact resistance.
Fixing of a stack of fuel cells by clamping is in widespread use. End plates provided at the ends of the stack are connected to one another via tie rods running along the stack and exert pressure on the stack, which holds the individual elements of the fuel cells and the fuel cells themselves in position in the stack.
The pressure required is considerable:
firstly, the fuels have to be passed through the fuel cell at a pressure which may quite easily be 2 to 3 bar.
Then, the seals have to be held under pressure, which likewise requires a pressure of the abovementioned order of magnitude of another 2 to 3 bar.
Finally, the contact resistance of the outer plates (generally graphite plates) is directly dependent on the contact pressure, which leads to the latter being very high.
The result of this is that in a conventional stack of fuel cells with 4 to 6 tie rods, each tie rod introduces a force of 104 N into the end plate, which leads to the required compressive load over the cross-sectional area of the stack of, for example, 100 to 300 cm2.
This compressive load should be as uniform as possible, since the majority of the cross section of the stack, i.e. of the surface area of the outer plates or the membrane of the individual cell, is (has to be) available for routing the fuels for the electrochemical reaction where substantially uniform conditions are present. Different requirements may apply in the edge regions of the cell, e.g. at cutouts for the tie rods or at the passages for routing the media to the individual cells.
As a result, a defined uniform compressive load is required over the cross section of the stack of fuel cells, as will be provided by the person skilled in the art for the corresponding structural design of the cell. As has been mentioned, therefore, in the context of the invention, a “defined uniform compressive load” is to be understood as meaning a load which does change, including a load with sudden jumps in pressure, but on the proviso that the change in load or any sudden jumps in pressure are defined in a desired way by the person skilled in the art, so that every region in the fuel cell is subject to optimum pressure. Apart from special applications, however, it will be the case that only minor changes in load are desired and there will be no sudden jumps in pressure.
The clamping forces which are introduced into the end plate at the edge sides by the tie rods cause the end plate to bend with respect to its bearing surface on the stack, with the result that the stack, as seen in cross section, is subject to high compressive loads at the edge sides and to only light compressive loads in the center, which contradicts the desired, defined uniform loading.
Consequently, end plates are often designed as solid, heavy elements reminiscent of armor plating. In particular a high weight and a high consumption of material are undesirable if it is to be possible for the stack of fuel cells to be used in mobile applications, such as for example vehicles, aircraft, etc., or if they are to be kept portable in general.
The prior art has disclosed numerous embodiments of end plates by which the desired, defined uniform compressive loading of the stack of fuel cells can be achieved more successfully at a reduced weight or deployment of material.
For example, WO 95/28010 shows a stack of fuel cells which are rectangular in cross section and having in each case four tie rods acting on one set of opposite sides, while two brackets, the ends of which are likewise subjected to load by tie rods, are provided on the opposite set of sides.
U.S. Pat. No. 6,428,921 shows a stack of fuel cells which are rectangular in cross section with tie rods running along the corners and acting on a double end plate. The outer plate has threaded bores into which bolts can be screwed so that they are then supported on the inner plate. Under operating load, the inner plate is prevented from bending outward, since screwing the bolts in sufficiently applies load in the inner region of the inner plate.
US 2002/0110722 shows an end plate with a set of springs arranged in the inner region on its side facing the stack; this set of springs exerts compressive load on the inner region of the stack even in the event of bending of the end plate under the clamping forces acting via the tie rods.
JP 9-259916 shows an embodiment with brackets which are subject to load from the tie rods, run in the inner region over the end plate and act on the end plate via local bearings.
The result of these embodiments is that the compressive load acts to an increased degree not only in the edge regions of the stack but also in the inner region. However, despite the conditions being improved, a defined uniform loading is scarcely achievable, and consequently locally elevated compressive forces still have to be used in order to maintain a minimum pressure at the locations which are not subject to direct compressive loading.
U.S. Pat. No. 6,040,072 shows a double end plate, the outer plate of which is thickened in the center but thinner at the edge sides, so that it bears centrally against the inner plate. Under operating load of the tie rods, the result is deformation of the outer plate, in such a manner that it then bears against the entire surface facing the inner plate in one plane, resulting in an improved compressive loading of the stack.
It is known to calculate the change in thickness of the outer plate by numerical methods with a view to achieving the desired, defined uniform compressive loading. Modern machining methods allow sufficiently accurate reproduction of the nonuniformly curved surfaces of end plates of this type.
For example in the case of a stack which is rectangular in cross section, the curvature may take place in one but preferably two directions (corresponding to the sides of the stack).
However, a drawback which remains is still the high weight of an end plate of this type, since the thickness required remains considerable. In the abovementioned example of a stack of 20 kg, some 2 kg is attributable to the end plates, even though they are made from aluminum. The accurate machining of the surface contour is also complex, in particular for series production.
A fin structure makes it possible to reduce the weight of an end plate of this type, but this further increases the outlay on machining. Numerical calculation models lead to structures which come very close to the desired compressive loading of the stack with the weight reduced still further. However, the outlay involved in producing structures of this type is huge.
Accordingly, the object of the present invention is to provide an improved end plate for a stack of fuel cells which allows a defined uniform compressive loading of the stack under operating load.
This object is achieved by the end plate, the pressure plate and the bearing plate described herein.